Electrostatic image development



March 15, 1960 c. F. CARLSON 2,928,575

ELECTROSTATIC IMAGE DEVELOPMENT Original Filed Oct. 12, 1953 4 Sheets-Sheet 1 i -FIGI 2 I INVENTOR CBZMI v-TCZALW BY Q ATTORNEY March 15, 1960 c. F. CARLS ON 2,928,575

ELECTROSTATIC IMAGE DEVELOPMENT Original Filed Oct. 12, 1953 4 Sheets-Sheet 2 IIIIIIIIIIIlIliIIII GLAa-F&,,Lw kahg-lj) E'fTORNEY March 15, 1960 c. F. CARLSON 2,928,575

ELECTROSTATIC IMAGE DEVELOPMENT Original Filed Oct. 12, 1953 4 Sheets-Sheet 3 FIGS INVESTOR WE E ATTORNEY March 15, 1960 c. F. CARLSON 2,928,575

ELECTROSTATIC- IMAGE DEVELOPMENT Original Filed Oct. 12, 1953 4 Sheets-Sheet 4 FIG-4 INVENTOR United States Patent ELECTROSTATIC IMAGE DEVELOPMENT Chester F. Carlson, Pittsford, N.Y., assignor, by mesue assignments, to Haloid Xerox Inc., Rochester, N.Y., a

corporation of New York Original application October 12, 1953, Serial No. 385,314,

now Patent No. 2,876,737, dated March 10, 1959. Divided and this application July 30, 1958, Serial No. 751,940

11 Claims. (Cl. 222-193) This invention relates to apparatus for the development of electrostatic images.

This is a division of my co-pending application, Serial No. 385,314, filed October 12, 1953 and now Patent No. 2,876,737 and entitled Electrostatic Image Development.

ln electrostatic image processes, such as Electron Photography disclosed in Carlson Patent 2,221,776, issued November 19 1940, and Electrophotography disclosed in Carlson Patents 2,297,691, issued October 6, 1942, and 2,357,809, issued September 12, 1944, the electrostatic latent image which has been formed on a surface is developed by bringing a finely-divided material, such as powder, or a liquid mist, adjacent to the image to produce a deposition of material on the surface or on a closely-adjacent surface to form a visible image. some instances the powder may be sprinkled or tumbled over the surface to be developed and excellent results have been obtained by a cascading method using a two component powder-carrier mixture as described, for example, in Wise Patent 2,618,552, issued November 18, 1952. Such methods have been most successful Where contrasting images are to be developed, such as black lines or letters on whitebackground. Where continuous tone images are to be developed, and also where large dark areas are present and even in some cases where high definition of line images is required it has been found that the most sensitive development method is one in which an air-suspension of finely-divided material, such as powder or liquid mist, is blown past the imagesurface. An important further measure of control of development can be obtained in such cloud development apparatus by providing a development electrode facing the image surface, as disclosed in application for U.S. Letters Patent, Serial No. 500,207, filed August 27, 1943, by Chester F. Carlson, now U.S. Patent 2,551,582, issued May 8, 1951, for Printing and Developing Solvent Images, and in divisional application Serial No. 166,411, filed June 6, 1950, for Electrophotography, now Patent No. 2,690,394. The development electrode can be electrically biased to a desired potential or series of potentials during development in order to control the deposition of material.

With liquid developers, such electrodes may be substantially self-cleaning or self-draining. However, with powder clouds it has been found that powder accumue lates quite rapidly on the development electrode necessitating frequent shut-downs for cleaning, or requiring awkward wiping mechanisms.

One feature of the present invention comprises an improved developing mechanism for developing electrostatic images on flexible sheet material by means of powder clouds and development electrodes and in which convenient cleaning means are provided for the development electrode. The development mechanism preferably comprises cylindrical surfaces around which a web of sheet material can be drawn and a cylindrical development electrode which can be rotated inside the channel formed in 2,928,575 Patented Mar. 15, 1960 the web, and cleaning means co-operating with the de velopment electrode.

Another feature of the present invention comprises the powder cloud generating and transportating means wherecloud development certain parts of the apparatus and certhe open top of the chamber.

tain features thereof are also applicable to other cloud development methods, such as liquid mist development.

Further aspects and novel features of the invention will be apparent from the following detailed description of certain embodiments thereof, and by reference to the appended drawings in which:

Figure l is an elevation, partly in section, of a powder cloud development apparatus for developing electrostatic images on flexible sheets or webs;

Figure 2 is a vertical section through the development head;

Figure 3 is a section on the line 33 of Figure 2;

Figure 4 is a section on the line =44 of Figure 2;

Figure 5 is a detail of a modified guide for the sheets vent cylinder 11, and a powder cloud generator 12 for introducing a powder suspension into the air stream. A powder cloud conduit 13 leads from generator 12 to development head 14 through which a web or sheet 15 carrying an electrostatic image is drawn for development by ,the powder cloud. The air stream and residual powder remaining in the stream after passing through the developing head is returned to the intake part of blower 10 by conduit 16.

Blower 10 may comprise an electric motor-driven rotary vane-type blower such as is used in household vacuum cleaners of various sizes. In one embodiment a 200 watt blower generated a powder cloud stream adequate for developing an 8 /2 inch width of image surface at web speeds of 20 to 60 feet per minute.

The air stream from blower 10 passes through conduit 17 into the base of air vent chamber 11 which consists of an upright cylinder having a closed bottom and a porous filter cloth or other air filter layer 18 secured over A plate or disk 19 is pivotally mounted at one edge of the top of cylinder 11 to be swung over the top by handle 20 to close a portion of the filter surface and increase the air resistance of the vent. It will be apparent that such powder as collects on the inside of filter 18 can fall by gravity to the bottom of cylinder 11 where it can again be picked up by the air stream. Except for the small proportion of air which passes out through the filter the remainder of the air stream and returning powder are carried through cylinder 11 to conduit 21 which leads into the base of powder cloud generator12.

Powder cloud generator 12 comprises an upright cylinder 22 which is closed at the bottom except for air desirable-finer particles are" carriedbyfthe airstr'eamtu 3 'fi'tte'cYwith' a screwcap' seaPZS isalso provided in the top'wall 23 beside'conduit 13;

Inside the base of generator 12 inlet conduit 21 extends to a point near the center where it turns upward to pro: vide a'central vertical -standpipeporticn 26? AwalI-Corii? swea ers preferably triboeiectrically related to the developer powder 31 in" suclr a" manner" asto irnpart electrostatic" charges of the desired polarityto the powderparticles to enable the powderparticles to be attracted to the electroprising an invertedcone 27 is secureti over thetdjafof standpipe 1 portion 26" and extendyout to "the side""wall 22: r

to which it*is' secured; The-apex" of theconeis afith e axis of cylinder'22- and since the'wall-of the=cone'slopes downward and outward to "cylindricalwall22 it formsan annular and substantially v-sha'ped trou'gh 30 in which' powder may settle fromthe-upper portions" of the-gen crater. It is" preferred; that thecone wail makeani angle withthecentral", axis' of 45 degrees on lessto facilitate sliding of powder towardthe outer, edgesofthe' trough.

Several tubes 28 are secured in the cone near its? apex and' extend' horizontally and radiallyto positionsspacedf circumferentially inside wall 22 where'theyare provided with constricted nozzles 29 pointed slantin'gly downward to eject air" into V-shaped powder channel 30; The? I static-images to be develop ednon; web 15. Thus, if the images. are positively cheirgdthe powder is preferably negativelwcharged and vice versar a Insomecasemghowever, it may be desirable to develop the images with T powder-bf are-s me charge polarity to obtain a r'eve'rsal Edward-:N Wise but for the purposes of the present inventionlthe proportions of carrier arei muchclcss thet nozzles areall-pointediu a cloclwise'directionas viewed from above so as" to produce a circular drifting or swirl ing ofthe powder 31 inehanneFSQ-as it is"pickedup by the air streams ejected from nozzles-29i From-two to" eight ormor'e' nozzle tub'es=ZWaremounted im cone- 27,

to conduct the air and any entrainedpowder frorri'stand pipe" 26* info the powder cloud: generating section of the I generator; The total cross section of urembes zs maya be about equal to that of each of the conduits-'13} 16;;17 and* lrbutifis" preferredthafthe nozzles" 29 bewonstricted slightly, at least so that the air or gas strea'rns issuinggfiorn'tthe nozilesare acceleratdfand '='able= to rai'se a substantial amount ofipoWdi- 31' resting? in chan nel flfl into"the air" stream and cr'eate a owdee clouds powder as, falls-back into the channel drifts=arouri in? clockwisedirection an'dFsoon corri'es uiider the streahtfof the next succeeding nozzle around the channel? The nozzles are spaced closely enough so' that' the drift fromone nozzle'is brought under thenext one and henceno chance-is provided for driftsof powdertrr fi rirri' and liedormant. a

Within generator wall 22 there is provided a ve'rtieal cylindrical bin wall 32 t which is-*open= at 'the rep and} bottom; The lower edge of wall 32% is slightl'y'spaced abovejfliewail of cone' 27 4 so that powder= can slide dowii; theco'ne"into*the' channelitm and the'f up peredge ofv'vall' carrier amountingto onlyv about 5% to by; weight;

7 Also slightlyv smaller carrier. particles are preferred; such as]. particles which pass through 60 n1esh and are-retained:

on 101) mesh, or in some'cases even including particles I between. IOQ-me'Shand QQOjrnesh, so; that they. maybe readily agitated and moved by, the high speed. air jets, issuingirorhnozzles p Q r The -interior walls .of" generator 12' and oficondiiitt 13'. and'idevelophientheadd imay be coated, if desired; iwitlfl filrnsl', ofimaterial havihg'the desired't triboelectric proper? ties to iinpart -the; correct. polarity, of charge to; the;

powder. ara es Inmany;cases thegstrueturallmaterial,

' itselfgwill have. the desi'rledieifec'tl and nof. coating Willi-be necessary Thus many 'resinou'smowd is; become; negae eiO an'dQin sectiouinEigures-lgilandt ir The powd'encloudi V 32 is preferably at or slightly below the'level of loweri end arm-name 13* which extendsdbwn a shot distance helbwuop w'all 23 of* the generator: Wall 32'; spaced fiom wall 22 sufiieiently-to-m'ake the crosssection annular space 33*gre'aterthair'that of any 'of conduits? 16; 17* and '21 and i preferably twice as great or mere Nozzles 29 are located in this-annular-space-and as the powder cloud is generatedlit'brises in this' sp'acefwitli a clockwiseswirling motion: 7

Heavypowder particles and agglomerates fallback in: toth'e channel partlybec'ause of some'c'entrifu'galsepara''-* tion': which ta-kesplac'e and partly because" of th'e*fact that thecrosssection'of space334s larger than the-con duits andf'hence' the slower speed of} the risingair' stream is insufficient to" support heavierfparticlese Gui? th oy'en-the top ofwalhfi ajnd'o'utthrough'conduit id 'kny a'gglomerates which' rnay rem after the stream passes? over wall 32 have afiirthe1-' opportunity to same warez:

The generatormay bereplehished' with p'o' used maY be any ofthose comr'nonlyfiuseti for powdeu maysbeemaintaiti'e'dain thetgeneratdrz These -nantielest are 75;

V r 1 5 time to time 'by removing screw c'ap -2 5=and*iiitroduciingi tiyelyh cliairgedg' by. contact or, rubbing; against many; common-metals.

CBindiiitlISgkalso returmcondilit 1 6:- andcondhitslfl and; be oren-tirely. formed. of flexible .rhaterial}; suchrubber-tor plastic hose to enable convenient mounting-toil thezdevelopingyheadr 14 inrelation: to gem erator 1'2' and; blower-I0; The flexible conduitalso helps; toisolate vibrations generated by blower 10ffrom the rest ofithe nsystemn t I a Bevelopmentheadlii.showntin side view iriEigure 'l r ri'sing throughoonduit,ISIfiIsLenters: a distributor, section 3minwhich? the: streanris spread=- or. distributed in; a

seriesgof;step :i nto. a&lamina the widihnof gthe image. area on-g,siie e t oi webr to-v be} developed; and}, preferablyslightly g wider.'thanl theaimage area." The distributor; ismade f a stack OftP18t6-35 tightly clampedvtog ethe'n by. bolts-56,; Sfiz and 38 -an'd provided with. cut-outi apertures whiclrrconnect withithoserin adjacent platesrtoprovide ia zigzag path: 39 fora the powder cloud; as seen most elearly;in-Figure-.-2-: The path-:graduallyi widens as itpasses upward as seen. from Figures 3' and- 4.- The lowermost plates 35, ,wherethe powder: cloud path is stilhnarrow;may, be -thickerthan ithose above to. preserve at relatively constant: cross section: for the= passageas. it

rouawa its: zig zag upward coursea The averageqcrosst section of the passage in distributor 34 may he substam tia'lly equaltto theorem-section ofitheapassageein conduit 15301; it: mawbesiightly' less; imordeni tO?aCCB lBIatE"-ther I streamislightlytandtredilcerthezpossibility of clogging-,1 in

' u 75,54, ut ofthe air stream into the central tin: provided by theidistribjutorisectionl fi a I l he:shapef+ofzthefpassage SQEmayrBe-iurmenunderstdod by reference to Figure 4 whichtis. fiseetionrtakenhoria- I e0'lit pifit'ci135 10bliing enerates dir ttre seet-ib'n line -hr Figure 2f "rheipas'segei-fid stans atthewntlet of condiiiris anw are cutward at a uniforr'rfrate until 'it reachesan arcuate elongated;apertureetzainttlieg nexplate 455; after which it again reverses direction and so forth. Apertures. 40 and 42 are shaped to contribute to the uniform distribution of the powder cloud stream across the width of the path. Thus, aperture 40 comprises an arc having its center at the intersection of the extension of the lines of the sides of the passage cut in plate 135. Aperture 42 may also have an outward curvature centered at the intersection of the lines of sides 41 of the passage in plate 335, or it may have a lesser curvature or be straight. This and subsequent apertures may also be varied in width along their length to aid the spreading of the powder cloud. Thus, they may be constricted slightly in the middle, as shown for aperture 42, to deflect the stream outward toward the ends of the slot. After the stream has passed through the first two or three apertures, such as 40 and 42 it will have reached the full width-of the image area but it is desirable to convey the stream through two or more further reversals in its zig-zag path'to permit further equalization to take place in the speed and density of the powder stream across its width. The walls of the passage in distributor 34 also contribute to the breaking up of powder agglomerates due to turbulent regions, and to the charging of the powder particles due to contact with the walls of the passage. Plates 35 may preferably be made of conductive material, such as aluminum or other metal. Insulating films, if present, are preferably of negligible thickness to prevent the building up of high electric charges on the inside walls. In some cases, however, the walls may be treated with thin insulating coatings of the desired triboelectric properties to impart charges to the powder, or with conductive coating for the same purpose. I

U The flattened powder cloud stream passes from the top of distributor 34 to a bypass valve chamber 43 (Figure 2) where the stream is divided into two flat streams consisting of a developing stream, which passes upward in a flat passage 47 between vertical walls 48, and a by-pass stream which flows through a passage 49 toward the right, as viewed in Figure 2, and joins the returning spent developing stream 50 returning from the image developing zone. A n adjustable control valve 44 comprising a wedgeshaped vane on a circular shaft 45 is located in valve chamber 43. The shaft can be rotated a few degrees by manual control handle 46 (Figure 3) to bring valve vane 44 against either of the flaring walls of chamber 43 to divert the stream entirely to either passage 47 or 49 or the vane may be set at any positionbetween these limits to divert a greater or lesser proportion of the powder cloud stream into by-pass passage 49.

The slope of wall 91 which co-operates with vane 44 to form the entrance to by-pass passage 49 is substantially parallel to the direction of the powder cloud stream as it enters valve chamber 43 while the development passage 47 turns upward within the valve chamber. Heavier particles and 'agglomerates, which may be present in the stream at this point are carried by their inertia into bypass passage 49 whereas the smaller and lighter particles, which are most desirable for development, are readily carried along with the air stream and are divided between the two passages substantially in the same proportion as the air itself is divided. This provides a developing stream containing mainly or exclusively the finer developer particles, while the larger particles and agglomerates are by-passed to the return passage.

Developer passage 47 extends vertically to its outlet 51 which comprises a horizontal slotfrom which the developing stream of the powder cloud emerges in an upward direction. A cylindrical development electrode 52 having integral web guide flanges 53 at its ends is mounted for rotation on a shaft 54 supported in hearings in end walls 55 of development head 14.

The axis of shaft 54 and cylinder 52 is parallel to the outlet slot 51 for the powder cloud and is to the right and-slightly above the slot outlet, as viewed in Figure 2.

6 s Walls 48 of the developer passage 47 are tapered or curved inward near the outlet-and the outlet slot is positioned close to the surface of cylinder 52 so that the powder cloud is ejected tangent to the cylinder surface and substantially in contact with it.

A web guide roller '56 mounted on free-turning shaft 57 has its axis parallel to cylinder 52 in the same horizontal plane and the surface of roller 56 is substantially tangent with the flanges 53 at the left side of the cylinder as seen in Figure 2.

Web. 15, carrying an electrostatic image to be developed, passes down around roller 56 and up between the roller and flanges 53, then around the top of cylinder 52 resting on flanges 53 so that the image area of the web is 's'pacedfrom the surface of cylinder 52 providing a developing space58 of approximately the same thickness as that of passage 47 which conveys the powder cloud to the developing space. For greatest effectiveness the developing space is made as thin as possible to bring development electrode 52 as close as possible to theshee't surface to be developed while still permitting passage of the powder cloud between the surfaces. In most cases the spacing is between 5 and mils, which means that flanges 53 will have a diameter of 0.01 to 0.2 inch greater than the diameter of cylinder 52. Roller 56 may be formed of slightly yielding material such as rubber to permit web 15 to be wedged between the roller and flanges 53 at the point of contact or a rigidroller can be used and shaft 57 bemounted' on a yieldingbearin-g sup: port- 7 The electrostatic image on web 15 is located on the surface which faces the development space 58. The web may comprise a flexible electrophotographic plate such as is described, for example, in aforementioned Patents 2,297,691 and 2,551,582. The electrostatic images, of course, are produced by electrophotographic techniques prior to entering the development apparatus. The web may also comprise a conductive foil carrying an electrically charged insulating image, as described, for example, in connection with Figure 11 in aforementioned patent 2,357,809. In other instances the web may consist of insulating sheet material, such as cellulose acetate, polystyrene, polyethylene or other plastic sheeting, or thoroughly dry paper with or without a plastic coating, on which an electrostatic image has been formed as disclosed in aforementioned Patent 2,221,776 or to which an electrostatic image has been transferred or applied by other methods.

As the powder cloud passes around developing space 58 between the surface of cylinder 52 and the imagecarrying face of web 15 the powder is attracted to the electrostatic image and is deposited upon the web to form a powder image. Due to the presence of closely-spaced conductive cylinder 52 the lines of force extending from the electrostatic image are largely aligned normal to the image surface and the electric field is concentrated in the developing space where it is eifective to attract powder.

In some instances further control of development can be imposed by applying a suitable potential to a curved electrode 59 which follows the curvature of the web as it passes around the development space. Electrode 59 is spaced close to the back of the web and is connected through commutator 65 to the sliding contact 63 of a' potentiometer 60. For the present we shall consider the commutator 65 as stationary so that electrode 59 is.per-. manently connected to contact 63. The resistance element of potentiometer 60 is grounded at its midpoint and cylinder 52 is also connected to ground. The ends of the resistance element are connected to the terminals of a battery 61 of several hundred volts potential. By ad-. justing the position of contact 63 on the resistance -,ele ment, an electric field is appliedbetween electrode 59, and cylinder 52, and its strength and polarity are readily controlled by the operator. p

In most instances a triboelectric charge is applied to the powder cloud during its generation which is opposite in polarity to the charge. of the electrostatic image so that the. powder is readily attracted to the image without, the aid of an externallyapplied field. However, frequently. the web may carry a s'mallchargei in. the. background.

' a es-M sisting entirely of insulating material. Where a conductive backing is provided onthe web electrode 59' may he brought into direct sliding contact with the backing to apply its potentialhirectly tothe conductive backing.

Where thisis not feasible cylinder 52- may be electrically areas which will give rise to undesirable background powder in the developed image... -1 33 "applying asmall field from electrode. 59 to cylinderSZwhich opposes. the

but it produces a heavier depositon the. image areas which is often helpful in making a weak image more leg bl Reversal development may also be efiected by. use of electrode 59. A powder cloud is produced having the same polarity of triboelectric charge as, the image areas. Hence the powder is repelled from the image. When a field is applied by electrode 59 which opposes the image field powder is attracted to the uncharged"background 7 areas to develop a reversal print. g The quality and character of continuous tone images can also be controlled by' placing potentials on electrode.

59'. It is often desirable to-vary the maximum and mi'ni-.

mum density of a print, to control the contrast, and-in some cases toaccentuate development of areas havi'nga given charge'density. To increase thebrightness of highlights contact 63may be set to a point'which will apply a field which cancels the fieldfrom the highlight areas. To

Electrode-59 is most suitably used with webs 15 cone insulated froni thellhead 14and various potentials applied to it while the conductive. web backing is held at ground 7 potential, or if no conductive backing is present. electrode 59 may be used at ground potential.

The web 15 is drawn; out of developing head 14 and downward and outward to the right (Figure 2) by a pair of co-acting rubberrollers 70., 71 .driven at the desired.

developing speed. In place of rollers 70, 71 the web may be wound upon a motor-driven take-up reel. In event the powder image is to be transferred to another surface the-web may be carried throughva transfer. mechanism where a web of sheet transfer material is. brought against it, 'or inisome instances transfer can be effected by feeding' a sheet or web of adhesive-coated material between. roller 71 and web 15i Between head 14 and rollers. 70;

71 the web passesthrough a fuser 72 which may be energized when-it is desired to affix the powder images directly to web 151' Tliefuser maycomprise an. electric oven having entrance and exit slots for the web, or it may be a chamber cohtaining the vapor of a solvent for the. powder image or for a plastic coating on the web. 1

. As web 15 isdra 11 away from flanges 53it passes. over the edge of enclosing wall 73 of head 14 andis spaced. slightly from it in the'image area to avoid contact with the powder'image. The spent powder cloud continues.

a down around cylinder 52 and then straight down through.

increase the overall density a field may be applied to aid 1 the deposition of powder over the entire web area.

' Toemphasize the darkerareas a different technique maybe employed, involving the use ofcornmutator' 65. The commutator has a rotary contact 6.6 connected to electrode59 and normally at rest on a stationary commutator segment 67 which is connected to potentiometer contact 63. The commutator is provided with a series of short contact segments similar to 67 and rotary contact 66' can be driven by motor 68, when energized, to sweep across the stationary contacts. Potentiometer contacts 62 and 64 are connected to other segments andrnovable conductive bridgingpieces 69 are used to connect adjacent segments where required to produce larger segments connected to any of the potentiometer contacts. By set.-

ting the positionsof contacts 62, 63 and 64 and placing is. applied toelectrode '59, for say 50% of the time,

which is opposite in polarity'to the image and sufiicient to. produce a field strength which will cancel or reverse thefield from all except the most highly charged image. areas. During this period of each cycle of commutator 65 only: thehighly charged areas are developed. For

another period; say of the cycle, the potential is lowered to permit some development of intermediate tones; along with further development of dark areas and finally, for the last 10% of the cycle thepotential' may be lowered still further to allow a slight fill-in inthe lighter areas. The commutator may complete a; cycle in one second or less and pass through several cycles during the return passage 50: where it joins the by-pass stream from passage 49 and the combined stream returns to the blower 10 through conduit 16. Distributor plates 35 are provided with aligned apertures 74 (Figures 2 and 4);.of

decreasing'length from top to bottom to converge the return stream into circular conduit 16. j a

While cylinder 52 maybe rotated by the drag of the web as it passes around flanges 53, it is often desirable to provide a positive drive. for the cylinder to, enable.

7 accurate control of the web'speedinthe developmentzone and to avoid imposing a dragon other parts of the equipment. As seenin Figure 3, shaft54 on which cylinder 52 is mounted," can be driven by motor'75 through speed reducer 76,, which may include a one-way clutch 92 to enable the webto be pulled forward independently of motor 75' when the motor is not needed, and also during initial threading of theweb.

As previously stated one of the principal drawbacks of development electrodes in the past has been the need for frequent shutdowns for cleaning. The development electrode cylinder 52 here provided is cleaned duringeach. cycle of rotation. A thin blade 77 is supported on frame member 78 to'slide'lightly against the surface of cylinder 52 where it descends away from the devel-.

oping space. Any largeaccumulationsof powder are scraped lightly from the cylinderand fall into the returning spent powder/cloud stream. Any thin powder deposit-still remaining on the cylinder is thenbrushed oif the cylinder by a brush 79 supported on spring. arms 80 inside a chamber 81 below the cylinder. A cleanout hole .82 .is providedin the end wall of the head at the bottom: of chamber 81 for' occasional removal of accumulated powder. I v

Blade 77' ma'ywbe formed of thin steel or Phosphor bronze shim stock. Brush. 7-9-may be a-fine bristle brush,

travel of a given point of, the'web through the develop 7 ment zone. Other forms of development can alsobe achieved with the cycling system... including reversal development.

plusln-or a sott felt pad.

By virtueof the expulsion-of a small amount ofxair through filter 1811there is a slight. suctioniproduced. at the places whereweb'15 entersrand leaves developing head 14 so that a slight intakeofiair' is produced andloss of powder cloud; is prevented or. minimized.

Figures .5. and 6. illustrateamodifi'cation of the: deveb opment: headto provide emote positive support for web 15 and independent rotation of the developmenf'elec trode. Web 15 passes around the development zone on" a pair of rollers 83 which are free to rotateon shaft 84 Y of developing electrode cylinder 85. A pair of endless rubber or fabric belts 86 pass around guide pulleys 87 then down around pulleys 88 and up between pulleys 88 and rollers 83 where web 15 is brought against the rollers. The belts then-pass around the rollers 83 resting on'the edges of web 15 and securely holding it against the rollers. The belts then pass around rollers 89 at the point where web 15 leaves rollers 83 and the belts return over pulleys 87 to complete their cycle. The web may be advanced at any desired speed by take-up rollers 70,.71 (Figure 1) independently of the motion of cylinder 85. Cylinder 85 is driven by gear 90 on shaft 84 from a motor and speed reducer drive, as previously described. This makes it possible to adjust the web speed and electrode speed independently for most efficient operation.

A powder cloud development mechanism has been described which provides for efiicient and continuous generation of a powder cloud, triboelectric charging of the particles thereof to a desired polarity by contact with carrier particles or with the walls of the generator, or both, separation of large particles and agglomerates, and breaking up of agglomerates by turbulence. The system also provides for spreading of the powder cloud into a uniform lamina the width of the area to be developed, the controlled by-passing of any proportion of the powder cloud to vary the intensity of the developing stream and also to remove further large particles and agglomerates from the developing stream, and the return of spent developer and by-passed material to the powder cloud generator.

Provision is also made for control of development by a conductive development electrode and by potentials applied to said electrode or to a control electrode spaced from the development electrode. The mechanism provides for continuous development of web or sheet material and for continuous cleaning of the development electrode.

Means are also provided to maintain a net suction on the development mechanism to prevent escape of powder cloud out of the system.

Other features include independent control of web speed and speed of cycling of the development electrode. Further features are apparent in the preceding specification and in the appended drawings.

It is apparent from the principles set forth in this invention that certain modifications can be made without departing from the spirit of the invention. For example, it is contemplated that cloud return passage 50 in the developing head can be provided with a zig-z ag path similar to entering passage 39 to converge the stream uniformly into return conduit 16.

Although the system has been described as an air system it is apparent that other gases can be circulated and where the word air is used in the claims, gases in general are intended.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A powder cloud generator comprising a vertical cylindrical chamber, a source of air pressure, a plurality of nozzles fed thereby located at angularly-spaced intervals around the inside of said cylinder and directed circumferentially to create a swirling air flow within the base of a said cylinder, means for introducing powder into said cylinder, and a powder cloud outlet near the top of said cylinder.

2. A powder cloud generator as claimed in claim 1 in i which a powder bin is provided having a discharge opcfi f" ing within said cylinder near its base whereby powder is discharged from said opening into said cylinder when the level of powder falls below said opening.

3. A powder cloud generator comprising a'vertical cylindrical open-ended member positioned concentrically within a cylindrical chamber enclosed at the top and bottom, said cylindrical open-ended member being spaced apart at its top and bottom from said chamber, means to create a swirling flow of gas around said open-ended member and between said open-ended member and cylindrical chamber, and output means at the top of said cylindrical chamber adapted to feed out of said chamber a mixture of powder particles in gas.

4. Apparatus according to claim 3 including means to feed powder from within said open-ended member into the path of the swirling flow of gas around said openended member and between said member and said chamber.

5. A powder cloud generator comprising a vertical cylindrical chamber enclosed at the top and bottom, a plurality of tubes positioned and disposed within said chamber each terminating in a constricted nozzle, each of said nozzles being positioned downwardly slanting in said chamber and in the same rotational sense and near the wall of said cylindrical chamber, means to feed gas under pressure to said tubes in said chamber and through said tubes to and through said nozzles to create a swirling air flow within said chamber, and output means from said cylindrical chamber.

6. A powder cloud generator separator combination comprising means to generate a powder cloud including a gas supply, a powder supply and means to feed said gas supply to said powder supply, means to subject a generated powder cloud to centrifugal action, means to feed a mixture of fine particles in gas to an output zone while substantially excluding coarse particles from said zone, and means to withdraw from said output zone a mixture of fine powder particles in gas for xerographic powder cloud development.

7. A powder cloud generator comprising a vertical cylindrical chamber enclosed at the top and bottom, means to feed gas under pressure into said chamber, an inverted cone positioned in said chamber and at the base thereof with its apex at about the axis of said cylindrical chamber, a plurality of tubes positioned and disposed near the apex of said cone and extending outward substantially horizontally and substantially radially within said chamber, a constricted nozzle positioned at the end of each tube positioned downwardly slanting and angularly to said tubes, said nozzles all pointing in the same rotational sense, means to feed the input gas to said chamber to said tubes and through said tubes to said nozzles, and output means from said cylindrical chamber, the path of gas fed into said chamber including passage through said tubes and then through said nozzles and into the channel formed between said inverted cone and the walls of said cylindrical chamber to create a swirling flow of powder positioned within said chamber and then out said output means.

8. A powder cloud generator comprising a vertical cylindrical chamber enclosed at the top and bottom,

means to feed gas under pressure into said chamber, an inverted cone positioned in said chamber and at the base thereof with its apex at about the axis of said cylindrical chamber, said cone having walls extending outward to the walls of said chamber, means for introducing powder into said cylinder and to the channel formed between said inverted cone and the walls of said chamber, a plurality of tubes positioned and disposed near the apex of said cone and extending outward substantially horizontally and substantially radially within said chamber, a constricted nozzle positioned at the end of each tube positioned downwardly slanting and angularly to said tubes, said nozzles all pointing in the same rotational direction, 

